It is known that the pilots of an airplane are assisted by numerous viewing screens mounted in the flight deck and on which the pilots can read data and measurements useful for navigating the airplane. In particular, these viewing screens make it possible to provide data relating to the meteorological conditions, to the flight itinerary and to the terrestrial relief.
Among the set of flight deck viewing screens, at least two screens are used to view meteorological data (turbulence, clouds, bad weather).
The first so-called “navigation” screen (known as the “Navigation Display”) makes it possible to provide meteorological data in a horizontal plane, that is to say at constant altitude. The horizontal plane, in which the meteorological data are monitored, is termed the “monitoring plane”.
The second viewing screen, the so-called “vertical screen”, makes it possible to provide relief data and meteorological data in a vertical plane, that is to say at different altitudes. This vertical screen takes the form of a rectangle whose left lateral edge is graduated vertically in altitude. A symbol representing the airplane is displayed on the vertical screen at the altitude of the latter, the pilot thus being able to easily observe whether bad weather is forming above or below the airplane. The airplane symbol can move vertically on the vertical screen. Indeed, in the climb phase, the airplane is for example situated at the bottom of the screen so as to improve the observation of the meteorological conditions above the airplane. On the other hand, in the descent phase, the airplane can be positioned at the top of the screen, thus improving the visibility of the relief and the observation of the meteorological conditions under the airplane.
In addition to the relief data and the meteorological data, the vertical screen also makes it possible to represent the position of the monitoring plane which is displayed on the navigation screen. Stated otherwise, the vertical screen makes it possible to visually represent the position of the horizontal plane for which the meteorological data are displayed on the navigation screen. For this purpose, a continuous horizontal straight line is displayed on the vertical screen at the altitude of the monitoring plane when a function, commonly termed the “elevation function”, is activated.
As a general rule, the altitude of the monitoring plane is modified automatically so as to correspond to the itinerary of the airplane. In order to anticipate possible bad weather, the pilot can modify the altitude of the monitoring plane manually. It is this function which is known by the term “elevation function”. The continuous straight line representing the monitoring plane can then be moved vertically on the vertical screen, the meteorological data displayed on the navigation screen then being updated as a function of the altitude selected for the monitoring plane.
This elevation function exhibits a major drawback when the monitoring plane is moved out of the limits of the vertical screen, that is to say to an altitude above that corresponding to the upper edge of the vertical screen or to an altitude below that corresponding to the lower edge of the vertical screen, it no longer being possible for the altitude of the monitoring plane to be represented by the altitude graduations of the vertical screen. The continuous line representing the position of the monitoring plane abuts against the edge of the screen, the edge of the screen and said continuous line then merging into one.
In order to more distinctly represent the position of the monitoring plane when the altitude of the monitoring plane is out of the limits of the vertical screen, the position of the monitoring plane is represented by a discontinuous and thickened straight line in proximity to the edge of the screen. Unfortunately, when the altitude of the monitoring plane is off-limits, there is a risk that the altitude of the monitoring plane will be wrongly interpreted by the pilot. Indeed, the pilot may err and interpret the discontinuous line as a continuous line, the pilot then being persuaded to monitor the meteorological data at the altitude corresponding to the screen's display limit.
Moreover, the pilot may omit to switch the elevation function to automatic mode after having modified the altitude of the monitoring plane. The pilot then thinks he is observing meteorological data at the altitude of the airplane, although the monitoring plane has been moved manually. The airplane may thus pass through zones of bad weather which were not anticipated by the pilot, the airplane then possibly being endangered.
Moreover, it frequently occurs that, on the navigation screen, one of the pilots monitors the meteorological data and the other pilot the terrain data. Now, it may happen that one of the pilots manipulates the elevation function without warning the other pilot, and this may give rise to a poor interpretation of the displayed data.